1. Field Of The Invention
This invention relates generally to an optical coupler and more specifically, to an improved optical coupler having improved performance at room temperature and a greater thermal operating range, and a process for making same.
2. Description Of The Prior Art
Optical couplers are well known and various methods have been proposed to produce and assemble such couplers.
Polarization maintaining couplers are a special case of single mode couplers and are generally made from polarization preserving or single polarization optical fibers.
Single mode fibers that are polarization sensitive are predominantly of two forms, one, a fiber that has a generally elliptical shaped core within a cladding such that a prefered axis for transmission is created and a second, that has a stress member or stress members included in the structure having different expansion coefficients to the cladding such that a stress is created with a distribution that causes birefringence such that fast and slow axes are created and the fiber becomes polarization preserving or polarization maintaining.
The latter birefringent fiber is predominant and the following disclosure refers to this type but is not limited to it as all fibers are affected by mechanical and thermal stress which cause birefringence and thus affect the polarization sensitivity of the fiber.
In order to make useful polarization preserving couplers, it is necessary to control the manufacturing and assembly processes that make the coupler halves and the optically contacted assembled coupler, with particular attention to stress that is initially present in the fiber or that may be introduced by the processes. Design must also consider factors that may influence stress.
The fiber is embedded in the substrate after careful orientation of the stress members such that they are symetrically placed with respect to the plane of the polished surface. Removal of cladding and stress member material changes the amplitude of the internal stress in the remaining fiber but does not alter the direction of the stress and similarly birefringent axes which are usually parallel and perpendicular to the polished surface. The foregoing is predicated on the fiber itself being symetrical and fiber quality control for this attribute is important for coupler yield in the manufacturing process.
Coupler halves, when assembled will have their birefringent axes parallel and coupling will be from fast to fast axis and from slow to slow axis without cross coupling of fast to slow axes. As the coupler has the polished fibers from two halves in optical contact and some external pressure on the substrate is required to make contact there is an external force acting on the fiber which adds to the existing internally generated stress due to the fiber structure. If the external stress is in the same direction as the internal stress, then polarization attributes are maintained. If the external stress is not in the same direction, then polarization preservation is degraded due to cross coupling. Securing the halves tends to maintain or increase the external stress.
Within the scope of the definition of "securing the substrates" in my U.S. Pat. No. 4,688,882, it is an advantage to eliminate the predominant method of securing with an epoxy resin and secure the substrates by fusing the glass at the edge without disturbing the fiber. This results in a coupler having one less component to consider for expansion coeeficient and increases the magnitude of thermal and mechanical shock tolerance. Implementation of fusing requires that the substrate material has a high tolerance to local heating and resulting large thermal gradient which is usually associated with a very low expansion coefficient. In the prior art, in one embodiment, a substrate may be designed where a skirted edge is presented for fusion. The lower mass of this edge and the low conductivity of the skirt to the general mass of the substrate allow a greater thermal gradient than a square edge.
Fusing can be implemented by local heating with infrared radiation from a filament placed in close proximity to the edge or by 10.6 micron CO.sub.2 laser radiation but in this case, the typical strong absorbtion in a thin surface layer must be modified by beam shape and intensity control. The electric arc and micro oxyhydrogen flames, as are common in fiber fusing apparatus, can also be used
A very low expansion coefficient substrate is in conflict with an advantage herein after described which shows benefit from a substrate that has an expansion coefficient that matches the effective coefficient of the fiber in the direction perpendicular to the optical contact surface. This conflict can be partially overcome by the design of a substrate shape that allows a large tolerance to different expansion coefficients as will be hereinafter disclosed.